I've read half a dozen books about WWI and another couple about the Russian revolution. I'm not sure if any can help you understand the clothing of the era any better, but if you're just looking for some general recommendations, maybe I can help.

The best "all encompassing" type of book I've come across on WWI is John Keegan's The First World War. The most interesting from a geopolitical perspective is David Fromkin's Europe's Last Summer. The latter is more interesting, because Fromkin attempts to explain how the system of honor and expectations of the nobility of old Europe came together to make war when none needed to be made. It's as much a psychological investigation of von Moltke (the chief of the German general staff) as it is a book about war. Fantastic read, and it will also have a great bibliography so that you can explore further.

My reading about the Russian revolution was sparked by my interest in WWI. If you have a lot of time and dedication, Richard Pipes' The Russian Revolution is an absolutely fantastic, but expansive text. It covers all the political strife in Russia from at least the mid-nineteenth century that came to a head in 1917, and extends through the Red Terror. I've just begun The Gulag Archipelago, but I'm not into it enough to make a rec one way or the other.